The invention relates to the measurement of particle velocities using Doppler techniques.
Wind speed determination is required in many fields such as, for instance, meteorology and aeronautics: for example, there may be a need to ascertain an aircraft""s true airspeed or its wake velocity. Known methods of determining wind speed use continuous wave coherent laser Doppler velocimetry. Airborne particles are assumed to be moving at the same speed as the air in which they are suspended. Thus, wind speed may be determined by measuring the Doppler shift in laser light reflected from the particles passing through a particular volume of the air. The particular volume is commonly called the probe volume. The Doppler shift is measured to a high accuracy by beating (xe2x80x9cheterodyningxe2x80x9d) the reflected light signal.
Monostatic laser Doppler velocimetry systems are so called because they have common transmit and receive optics. A beam of laser light is transmitted from the system to the spatial region within which the wind speed is to be determined. Laser light reflected from airborne particles back into the same optics is analysed to determine wind speed. Spatial resolution is achieved by focusing the transmitted beam into the spatial region. In practice, diffraction restricts monostatic systems to ranges of a few hundred meters. Moreover, slow drop-off in sensitivity away from the focus may result in a poorly defined probe volume. As a consequence, clouds, smoke or other objects away from the probe volume may result in the generation of spurious reflections. Thus, where precise localisation of wind speed is required for instance, in wind tunnel studies, monostatic systems are not ideally suited.
Bistatic systems, which derive their name from having separate transmit and receive optics, are better suited to localised wind speed measurements. The optical set up in a bistatic system means that non-parallel transmitted and xe2x80x9creceivedxe2x80x9d beams can be arranged to intersect, thereby providing a well-defined probe volume. Confinement of the probe volume is advantageous for the reasons stand above, although it may lead to a reduction in signal power: in general, higher range resolution is accompanied by a greater reduction in carrier-to-noise ratio. For satisfactory alignment, the transmitted and xe2x80x9creceivedxe2x80x9d beam foci must coincide at the intersection, which can be problematical to achieve.
Disclosed in U.S. Pat. No. 5,875,024 is a Doppler sensor apparatus having separate transmit and receive optics, and light from a laser source is coupled into the output of the receive optics for use in aligning the transmit and receive optics.
The invention provides Doppler sensor apparatus comprising means outputting coherent electromagnetic radiation, means for transmitting radiation from the source means to a point in space, means for receiving radiation reflected from the point in space, means for determining any Doppler shift in the reflected radiation, wherein the transmitting means and the receiving means are separate, and wherein the radiation receiving means and the radiation transmitting means are each adapted to have radiation guiding means releasably attached thereto, thereby enabling radiation guiding means for routing radiation from the source means to the transmitting means and from the receiving means to the determining means to be interchanged with radiation guiding means for routing radiation for use in aligning the transmitting means and the receiving means simultaneously to the transmitting means and to the receiving means.
The apparatus according to the invention provided a well-defined probe volume for Doppler sensing, and effective rejection of any scattered radiation originating from outside of the volume close to the point in space.
Having releasably attachable radiation guiding means enables the straightforward interchange of components of the apparatus facilitating the determination of Doppler shift and component failitating the alignment of the transmitting and receiving means. Hence, satifactory alignment of the transmitting and recieveing means may be achieved with the minimum of difficulty. In addition, the transmitting means and the receiving means may be remote from the remainder of the apparatus which may be of assistance for measurements in awkward or hostile environments. The more delicate or expensive components can be positioned in a safe location.
Preferably, the radiation guiding means is optical fibre means. The optical fibre means may be a single-made optical fibre.
Also preferably, radiation is guided between the optical components of the apparatus by optical fibre means.
Further preferably, the source means is a semiconductor laser. The semiconductor laser may be an external cavity semiconductor laser diode.
The apparatus may comprise first means for splitting the radiation from the source means between two first outputs. In the event that the radiation is guided between components by optical fibre means, the first means may be a fibre coupler.
The apparatus may also comprise an isolator, for transmitting radiation in one direction only. This can be used to eliminate the problems of feedback into the source, which can adversely affect noise levels. In the optical environment, such isolators are known as optical diodes.
The apparatus may further comprise an amplifier for raising the power of the transmitted radiation to a level sufficient to permit reliable detection of the reflected radiation. Again, in the event that the radiation is guided between components by optical fibre means, the amplifier may be an erbium-doped fibre amplifier.
The apparatus may also further comprise a modulator. In the optical environment; an acousto-optic modulator may be used to shift the frequency of the radiation output by the source thereby enabling discrimination between positive and negative Doppler shift.
The radiation guiding means for guiding radiation for use in aligning the transmitting means and the receiving means may comprise second means for splitting the radiation from one of the first outputs between two second outputs and radiation guiding means for guiding radiation from the two second outputs to transmitting means and the receiving means. Alternatively, the radiation for use in aligning the transmitting means and the receiving means may be obtained from a separate source.
The apparatus may yet further comprise a polarisation controller for matching the state of polarisation of the received radiation with the reference (local oscillator) radiation. Alternatively, polarisation preserving radiation guiding means or optical fibre means may be used.
The apparatus may in addition comprise means for combining radiation from the second of the two first outputs with radiation from the receiving means and for splitting the combined radiation between two third outputs.
The determining means may comprise monitoring means for receiving radiation from a first of the third outputs and detector means for receiving radiation from the second of the third outputs.
The receiving means may have first and second receiving inputs each receiving different reflected radiation from the point in space.
Aligning the transmitting means and the receiving means may involve using a beam profiler or, alternatively, using apertures and/or fluorescent cards.
The apparatus according to the invention is particularly suited to use as a continuous wave Doppler wind sensor.
The invention further provides, in Doppler sensor apparatus having means outputting coherent electromagnetic radiation, means for transmitting radiation from the source means to a point in space, means for receiving radiation reflected from the point in space, means for determining any Doppler shift in the reflected radiation, wherein the transmitting means and the receiving means are separate, a method of aligning the transmitting means and the receiving means comprising transmiting radiation simultaneously from the transmitting means and the receiving means and adjusting the position of the transmitting means and the receiving means until the radiation transmitted by each intersects.